Musnad Ahmad 30:1567

Ishaq ibn Isa narrated to us, Abu Awanah narrated to me, from Umar ibn Abi Salamah, from his father, from Abu Hurairah, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Three things are all a duty upon every Muslim: visiting the sick, following the funeral processions, and saying 'Yarhamuk Allah' (may Allah have mercy on you) to the one who sneezes, if he praises Allah, the Almighty and Majestic."